The advancements in microprocessing and integrated circuit technologies offer the opportunity to control a wide variety of pneumatic, hydraulic, mechanical and electrical systems. Such technology has also been developed in and applied to motor vehicles. The versatility of microcontroller systems makes them well adapted to control a wide variety of functions performed by the components and accessories of a motor vehicle. However, the electrical signals from sensors or to actuators communicated to or from the microcontroller must be buffered so as to form an acceptable input to or output from the microprocessor. As they are used in this application, the terms microcontroller or microcontroller system are used generically and include both systems having microprocessors without resident memory and microcomputers having resident memory.
A previously known form of buffer circuit comprises a transistor and resistors which are selected in accordance with the input o output signal to be transmitted or received. In particular, an NPN transistor Q1, as shown by circuit 110 in FIG. 4, may form an input buffer by coupling a resistor R1 between its base and the input terminal I1 at terminal 20, a resistor R2 between its base and its emitter which is also tied to the ground terminal 18, a resistor R3 between the power terminal 16 and the collector of the transistor, and a resistor R4 between the collector and the buffer output terminal 22 coupled to an input port IP1 of the microcontroller. Such an input buffer is useful when the input is switched to a positive voltage.
Another common type of input buffer uses a PNP transistor Q2, as shown in circuit 114 of FIG. 4, with the base coupled to the input terminal I2 at terminal 22 through resistor R4. Resistor R3 is connected between the base and emitter and the emitter is also connected to the power terminal 16. The collector is connected through resistor R2 to the ground terminal 18. The collector is also connected to the input port IP2 of the microcontroller at terminal 20 through resistor R1. Such an input buffer is particularly useful when coupling in a low side switch for switching to ground.
A common type of output buffer comprises a NPN transistor Q1 (FIG. 4, circuit 116) with its base terminal connected to the output port OP1 of the microcontroller at terminal 20 through resistor R1. The base terminal is also connected to the ground terminal 18 through resistor R2, its emitter connected to the ground terminal, the collector is connected to the power terminal 16 through resistor R3 and to the output terminal 01 at terminal 22 through resistor R4. This is an active pull-down output buffer or low side switch used where current sinking to ground is required.
Another common type of output buffer uses a PNP transistor Q2 (FIG. 4, circuit 112) with its base coupled to the output port OP2 of the microcontroller through resistor R4. The base is also connected to the power supply terminal 16 through resistor R3, and the emitter is connected to the power terminal 16. The collector is connected to the ground terminal 18 through resistor R2 and to the output terminal 02 at terminal 20 through resistor R1. This is an active pull-up type of output buffer and used when output is delivered to a device requiring current sourcing.
In view of the needs discussed above for both NPN and PNP buffer circuits for the microcontroller, the configuration of a generic circuit board can cause substantial packaging problems. The problems of circuit variety and associated packaging are particularly acute in motor vehicle systems, especially for automobile manufacturers having many model lines and options. In particular, it may be desirable for the circuit board to be designed to accommodate a generic input/output buffer circuit for each microcontroller terminal to be made available for any of a variety of uses. As a result, the circuit board can be configured for placement of the resistors R1, R2, R3, R4 as well as the PNP and NPN transistor. While the schematic diagram for such a generic buffer arrangement is quite simple as shown in FIG. 3, the layout of such a generic circuit arrangement substantially increases the size of the circuit board packaging.
The use of circuit board space has been only partly addressed by the fact that component packages mounted to circuit boards have been standardized. Standardization dictates predetermined size packages and corresponding placements of conductive pads arranged on the circuit boards so that each terminal of a standardized component package can be mounted in electrical contact with a pad regardless of the performance values of the component mounted in that position. However, generic circuit boards would allow the arrangement of an active pull-down input or output buffer, or an active pull-up input or output buffer, at each terminal to a microcontroller chip. Such construction would increase the size of footprint even though only part of it is actually used. For example, it has previously been necessary to provide both a first footprint with three pads for surface mounting of a standardized NPN transistor package, and a second, separate footprint with three pads for mounting a standardized PNP transistor package. Moreover, in many cases, each separate transistor footprint requires additional peripheral pads for mounting the resistors R1-R4 around the transistor footprint pads actually chosen to be used for a particular buffer circuit. Thus, not only the additional transistor footprint increases the size of the circuit board area used for the generic circuit which is necessary, but the surrounding pads for set up components also increases the area that must be occupied to build a single circuit on the generic circuit board.